1. Field of the Invention
The present invention is directed to pressure vessels and more particularly to a nuclear reactor pressure vessel nozzle seal.
2. Description of the Prior Art
The conventional nuclear reactor pressure vessel comprises a longitudinally disposed cylindrical structure, closed at both ends by a convex base and a domed roof, having reactor coolant inlet and outlet nozzles protruding therethrough. Generally, these nozzles are disposed in a plane transverse to the longitudinal axis of the vessel and angularly separated from each other. Housed within the pressure vessel structure are, among others, the nuclear core, subassemblies and a fluid coolant. Moreover, within the pressure vessel, an annular flange is formed on the inner surface thereof. The flange serves as a means for supporting the reactor core which is suspended from a distribution hoop or shell.
The distribution hoop is extended by means of a thermal shield-skirt assembly, which supports the fuel elements in the reactor core and which also serves as a hydraulic guide.
In operation, the fluid coolant, in forced circulation, enters the pressure vessel through the inlet nozzles, and flows through the annular hydraulic guide that is formed between the inner surface of the pressure vessel and the skirt. The coolant then rises through the core of the reactor whereupon it is discharged from the vessel through an outlet nozzle which is in fluid communication with the hoop opening through conduit means interposed therebetween.
To insure proper circulation, it is imperative that direct communication be prevented between the incoming coolant and the discharging coolant. Toward this end, a leak-proof contact between the hoop opening and the pressure vessel outlet nozzle is required. However, although a leak-proof contact is necessary to prevent direct fluid communication, structural and differential thermal expansion conditions which can occur between the internal reactor structures and the pressure vessel must be considered. In general, the attendant thermal expansion precludes fixedly joining the conduit means to both the hoop and the pressure vessel wall. Therefore, a leak-proof sealing means, either as part of the conduit or in substitution thereof, is required to prevent the commingling of the inlet fluid coolant and the outlet fluid coolant. Further, from a structural consideration it is desirable that the sealing means segregate the fluid coolants without structurally coupling the hoop to the pressure vessel.
In the past, a leak-proof seal was established by a spring biased contact of a sealing ring or by thermal expansion contact of the conduit. In general, the thermal expansion contact seal consists of carefully and tediously machining the conduit or a ring to be attached thereto to establish a designed clearance or tolerance between the machined conduit or ring face and the pressure vessel nozzle during assembly. The leak-proof condition, however, for this thermal expansion type seal is only achieved at the elevated operating temperatures of the nuclear reactor system when thermal expansion of the hoop and conduit expand to meet the inner wall of the pressure vessel. Moreover, since the pressure vessel also expands during operation, this thermal expansion conduit-seal generally requires a material having a greater thermal expansion coefficient for the hoop and/or the conduit than the expansion coefficient of the pressure vessel, if the leak-proof state is to be achieved.
The spring contact type seal, moreover, comprises a cylindrically shaped sealing member disposed within and extending from a cylindrical annular cavity concentric therewith. The sealing member is generally machined on one face of its cylindrical shape in order to nestle in close contact with, for example, the pressure vessel wall about the outlet nozzle and thereby prevent leakage therebetween. A spring disposed within the annular cavity interposed between the other face of the cylindrical sealing member and the rear wall of the cavity, or a compression ring, exerts in the axial direction the force necessary to tightly seat the sealing member against the pressure vessel wall. Moreover to prevent leakage flow from one fluid from traveling through the annulus, between the sealing member and the annular cavity, and across the spring into communication with the other fluid, both the sealing member and the cavity are machined to exact close fitting tolerances such that the sealing member is tightly seated in the cavity. However, the seating or mating of the seal ring to the cavity, even with the strict matching of machining tolerances of the slideably engaging members, produces a narrow gap therebetween. Moreover, although this narrow gap provides a labyrinth-like flow passage, the high differential pressures encountered within a pressure vessel enhance fluid leakage flow therethrough and establish fluid communication between the inlet and outlet coolants. Therefore, flow leakage or fluid communication between the coolants is not prevented but merely reduced by this type of seal. Moreover, from a cost efficient viewpoint, this leakage rate is too large and the machining tolerances are too stringent for economic justification of this type seal.
Accordingly, there is a need to provide a sealing means which will prevent or at least reduce the leakage flow between the incoming and discharging coolants at all operating conditions without the stringent manufacturing tolerances, or the use of different materials having different thermal coefficients that are characterized by the prior art systems.